264
MAKOTO OMORl'
possessing dcrmtrl photophores we larger in proportion to body size
than the eycs of other specios which lack photophores.
VL. SHOALING
The distribution of maiiy dirimps is uneven and patchy. Probably,
shoaling may rcsult primarily from the interaction of the shrimps
with each otjlier, although its mechanisms and functions are not well
known. Clutter (1969) studied the structure and function of a hypopclagic popultition of mysids. H is observations suggested that the
social bohirviour of the mysids could be interpreted as related to
maintaining position within their habitat, reducing predation, increasing probability of copulation, and regulating population. Nothing
is known about the social behaviour of the shrimps, but many species
which have itnage-forming eyes and a set of luminescent organs must
be able to congregate to facilitatle mating and regulating population.
In copepods and euphausiids the shoaling behaviour is significant
in herbivorous species living in the surface or sub-surface layers.
Although decapod crustaceans are not herbivorous when adult, penaeids
and sergestids feed on phytoplankton during the early larval stages.
Gut contents of Sergia lucens indicate that even adults eat some
diatoms in certain seasons. Post-larval pelagic phases of the genera
Jlunidn and P1euron.codes often show the capability of grazing, by
filtration, upon blooms of the l q e r phytoplankton organisms (Longhiirst et nl., 1867). These facts indicnt,e that the formation of vast
shouh tippears to be characteristic of fil ter-feeders or potential filterfeedern. Although the relationship between observed behaviour
and feeding habit cunnot be interpret,c!cl fully, the function of shoaling
in these kinds of animals, particitlarly by the larvae and juveniles of
the shrimps, seems likely to help control their population and to
maintain position within their habitat zone. As the population becomes larger, the shod which i R composed of swarms and schools
becomes larger. However, since the witable habitat zone is limited,
the density of the species becomes higher, and by physical contact the
individuals will always be able to relate their population size to the
amorint of food supply in their own zone. Then, regulat,ion of the Rhrirnp
population, if necessary, will be taken care of by cannibalism and
predation. Also retardation of growth of the whole population due to
lack of food may cause restraint of reproduction.
The meso- and bathypelagic carids are carnivorous throughout
life aiid are unlikely to form great shoals. Our samples indicate that
thcy Hometimes congregate in small swarms conrristing of a few individurtls. The distance between one swarm and another appears to IN
MAKOTO OMORl'
possessing dcrmtrl photophores we larger in proportion to body size
than the eycs of other specios which lack photophores.
VL. SHOALING
The distribution of maiiy dirimps is uneven and patchy. Probably,
shoaling may rcsult primarily from the interaction of the shrimps
with each otjlier, although its mechanisms and functions are not well
known. Clutter (1969) studied the structure and function of a hypopclagic popultition of mysids. H is observations suggested that the
social bohirviour of the mysids could be interpreted as related to
maintaining position within their habitat, reducing predation, increasing probability of copulation, and regulating population. Nothing
is known about the social behaviour of the shrimps, but many species
which have itnage-forming eyes and a set of luminescent organs must
be able to congregate to facilitatle mating and regulating population.
In copepods and euphausiids the shoaling behaviour is significant
in herbivorous species living in the surface or sub-surface layers.
Although decapod crustaceans are not herbivorous when adult, penaeids
and sergestids feed on phytoplankton during the early larval stages.
Gut contents of Sergia lucens indicate that even adults eat some
diatoms in certain seasons. Post-larval pelagic phases of the genera
Jlunidn and P1euron.codes often show the capability of grazing, by
filtration, upon blooms of the l q e r phytoplankton organisms (Longhiirst et nl., 1867). These facts indicnt,e that the formation of vast
shouh tippears to be characteristic of fil ter-feeders or potential filterfeedern. Although the relationship between observed behaviour
and feeding habit cunnot be interpret,c!cl fully, the function of shoaling
in these kinds of animals, particitlarly by the larvae and juveniles of
the shrimps, seems likely to help control their population and to
maintain position within their habitat zone. As the population becomes larger, the shod which i R composed of swarms and schools
becomes larger. However, since the witable habitat zone is limited,
the density of the species becomes higher, and by physical contact the
individuals will always be able to relate their population size to the
amorint of food supply in their own zone. Then, regulat,ion of the Rhrirnp
population, if necessary, will be taken care of by cannibalism and
predation. Also retardation of growth of the whole population due to
lack of food may cause restraint of reproduction.
The meso- and bathypelagic carids are carnivorous throughout
life aiid are unlikely to form great shoals. Our samples indicate that
thcy Hometimes congregate in small swarms conrristing of a few individurtls. The distance between one swarm and another appears to IN